1. Field of the Disclosure
The present disclosure relates generally to scan units for image forming device, and particularly to scan units that reduce or otherwise eliminate distortion due to use of both forward and reverse sweeps of a laser beam.
2. Description of the Related Art
The thermal, power, start-up-time and acoustic advantages of micro-mirror-based laser scan units of electrophotographic imaging devices are well demonstrated when compared with scan units utilizing a polygonal mirror. A drawback in using the micro-mirror is its relatively low scan duty cycle, i.e., the time the light beam illuminates the photoconductive member versus the time the light beam illuminates areas off the photoconductive member. This relatively low duty cycle is driven by the sinusoidal motion of the micro-mirror in the scan direction and the practical need to have the light beam velocity in the scan direction acceptably high as it moves onto the photoconductive member.
A further loss of potential duty cycle is encountered by the “zig-zag” trajectory of the laser beam on the photoconductive member if both the left-to-right (forward) and the right-to-left (reverse) sweeps of the reflected laser beam are used to produce the latent image on the photoconductive member. Scan units that use both the forward and reverse sweeps to create scan lines on the photoconductive member, and print defects due to zig-zag distortion can be demonstrated with specific print patterns. These print patterns are not likely to be used by many print jobs, so this potential print defect is less of a concern for monochrome devices. With the need to increase print speed and/or resolution, or the desire to otherwise apply the advantages of a micro-mirror based scan units for color applications, eliminating zig-zag distortion is advantageous. For multi-emitter architectures to increase speed and/or resolution, the advantage is primarily to reduce the extreme scan spacing overlap. For color applications, the primary advantage is to enable overlapping halftone screens needed for color applications.
The root cause of zig-zag distortion is the use of both the forward and the reverse motion of an oscillating mirror to create scan lines on the photoconductive member, coupled with the substantially constant rotational speed thereof. As shown in FIG. 1, the location A in the process direction at which the beam exits the photoconductive member in a forward sweep (beam location 110 mm) versus where it enters on the immediately following reverse sweep at location B is estimated to be about one half of the distance from location C where the beam enters on the forward sweep (−110 mm beam location) versus where it exits at location D on the following reverse sweep. This change in sequential scan spacing is further exacerbated by a 2-on-2-off line pattern where the darkness of the left edge, center and right edge of a page are different.
If one looks at the impact of zig-zag distortion in a multi-emitter case, the issues are multiplied in that, for a two emitter scenario, the separation between the forward sweep of a first emitter versus the reverse sweep of the second emitter can, depending on the scan efficiency, go to zero or even negative. This situation is illustrated in FIG. 2. As can be seen, along the 0 mm scan direction, beam spacing is substantially uniform but along the +110 mm and −110 mm locations, the beam spacing is not at all uniform.
Based upon the foregoing, there is a need for an improved scan unit having a bidirectional scanning oscillator.